1. Field of the Invention
The present invention relates to an exposure control technique in an image capturing apparatus.
2. Description of the Related Art
Conventionally, image capturing apparatuses such as an electronic camera and video camcorder generally perform exposure control by controlling exposure parameters such as the aperture value, shutter speed, and image gain based on the luminance level of a captured image. This method can determine the luminance level for an actually captured image. Thus, this method can advantageously increase the exposure control precision, change the weight depending on the image area, and perform accurate partial photometry.
As another photometric method, a camera using a silver halide film, a single-lens reflex type camera, and the like comprise an independent photometric sensor, and determine the exposure in accordance with an output from the photometric sensor.
To utilize the advantages of these two photometric methods, Japanese Patent Publication No. 5-49151 proposes a photometric method using the arrangements of the two methods. According to Japanese Patent Publication No. 5-49151, light is metered by an external photometric apparatus to determine an aperture value which provides a correct exposure amount. The electronic shutter executes exposure for an exposure time by which a correct exposure amount is attained on the assumption that the determined aperture value is proper. The exposure amount at that time is detected, and a correct exposure time is determined in consideration of the photometric error of the external photometric apparatus, the aperture error, and the error of the correct exposure amount. Then, exposure for shooting is done without changing the aperture value. Hence, high-precision exposure control can be achieved within a short time without correcting the exposure time again and again.
Japanese Patent Laid-Open No. 2001-36806 makes a similar proposal in order to complete pre-exposure within a time as short as possible in shooting a still image. According to Japanese Patent Laid-Open No. 2001-36806, when the first shutter speed obtained based on an external photometric value is lower than a reference value, the second shutter speed higher than the first one is set, and pre-exposure is done using the second shutter speed. This can shorten the time taken to execute pre-exposure, and decrease the time lag until the start of actual exposure.
However, the conventional image capturing apparatus suffers the following problems.
In the arrangement in which exposure control is performed based on the luminance level of a captured image, no problem arises when the shutter speed is sufficiently high and the output period of an image capturing signal is short. However, in shooting at slow shutter speed for a long charge storage time, the output period of an image capturing signal becomes long, the exposure detection period based on the luminance level also becomes long, and the response characteristic of exposure control becomes poor. That is, when the luminance greatly changes, a long time is taken until the exposure of an image is stabilized. In shooting a moving image, the exposure may not be stabilized depending on the output period of an image capturing signal, and overexposure and underexposure may be repeated.
In the arrangement in which an independent photometric sensor is adopted to determine the exposure in accordance with an output from the photometric sensor, the luminance level cannot be determined for an actually shot image, and the exposure control precision may decrease. It is difficult to change the weight depending on the image area and perform accurate partial photometry. Particularly when the zoom lens is used, the photometric range changes depending on the focal length.
According to the photometric method using these two arrangements, the conventionally proposed arrangement is effective for only still images, but is not effective in shooting moving images.